The present invention relates to a process for producing a glass and a glass composition with a compressive surface layer having a high strength, and more particularly to a process for increasing the mechanical strength of alkali silicate glass by an ion exchange where the glass is contacted with a fused salt of an alkali metal ion having a larger ionic radius than that contained in the glass, so as to replace the alkali metal ion in the glass with the larger alkali metal ion. Hereinafter this will be referred to as an alkali metal ion exchange.
Methods for increasing the strength of glass are known where the alkali metal ion exchange is conducted at lower temperatures - that is, at the strain point or below - to produce a surface layer having a compressive strain in the glass. In most cases, however, the replacement of the smaller alkali metal ion in the glass with the larger alkali metal ion in the salts proceeds at an extremely low rate and requires a considerable time for attaining a satisfactory thickness of the compressive layer. In the cases of conventional soda-lime-silicate and lead-silicate glasses the treatment is required to be carried out over a period of about 10 hours. Even when such a long treatment has been conducted, the bending strength of the treated glass is only 3,000 - 4,500 kg/cm.sup.2 and the depth of the formed compressive surface layer is scarcely ever more than 10.mu.; since the compressive layer formed is rather small in its thickness, as mentioned above, these types of glasses have disadvantage in that since the strength tends to be affected by fine flaws produced in the surface, the values thereof are distributed over a wide range, and that the strength is easily lowered due to fine surface scratches produced during customary handling after treatment. The degree of distribution of strength values as expressed by a ratio of a standard deviation to an average value of strength is referred to herein as a strength deviation. Conventional soda lime-silicate and lead-silicate glasses have strength deviation of 12 - 14% while alkali metal ion exchanged ones have much layer value (15 - 20%). Therefore, the glass of these types indicate unimpressive results by alkali metal exchange.
Although an attempt has been proposed in Japanese Pat. Publication No. 20629/66 for overcoming such disadvantage by increasing the rate of the alkali metal ion exchange using a glass having a high content of ZnO.sub.2, this type of glass has a high viscosity at melting temperature and is poorly workable.
According to this patent the surface compression strain is produced by the difference of heat expansion coefficient between the surface and inner portions of the glass. In the case that crystals of low heat expanaion coefficient are educed in the surface portion, it causes a large difference of heat coefficient and a high strength glass is produced. However, this glass has a defect in that it is relatively readily devitrified in its surface portion. On the contrary, when crystals of low heat expansion coefficient are not educed in the surface portion, its strength is lower generally below 3000 kg/cm.sup.2 , as the difference in the thermal coefficient due to the alkaline ion exchange is small. Furthermore, this method has a disadvantage that glass articles are apt to be deformed during the ion exchange process because the ion exchange is perfomed at a higher temperature than the distortion point of the glass composition so treated.
A glass having a high rate with respect to the alkali metal ion exchange, for example, is described in the publication, "Physics and Chemistry of Glasses" Vol. 5, pp 123-129, which discloses that the SiO.sub.2 - Na.sub.2 O-Al.sub.2 O.sub.3 glass composition having a ratio, Al.sub.2 O.sub.3 /Na.sub.2 O, of 1 or more is suitable. However, this glass composition has disadvantages in that it also is too viscous for a conventional melting process.
As a glass having a low melting temperature with a good workability and yet treated at a relatively high rate of the alkali metal ion exchange, Japanese Pat. Publication No. 3632/68 discloses the SiO.sub.2 -RO(CaO or MgO)-R.sub.2 O(Li.sub.2 O, Na.sub.2 O or K.sub.2 O) glass composition having at least 50 mol % of MgO on the basis of the total amount of RO. Further, as a glass having similar features, Japanese Patent Publication No. 19420/68 discloses that the R.sub.2 O(Li.sub.2 O, Na.sub.2 O or K.sub.2 O)-MgO-SiO.sub.2 -Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 glass composition is suitable. These glasses have to be contacted with KNO.sub.3 or NaNO.sub.3 at high temperatures for 4-20 hours. This period of treatment time is still unsatisfactory, although it is much shorter than those in the cases of conventional soda-lime-silicate and lead-silicate glasses. Moreover, the treatment requires large apparatus when a great number of articles are treated, another disadvantage. Further, Japanese Patent Publication No. 26055/68 discloses that the SiO.sub.2 -Al.sub.2 O.sub.3 -Na.sub.2 O-ZnO-F.sub.2 glass is treated at a high rate in the alkali metal ion exchange the resulting products have a markedly higher strength after the ion exchange treatment than those of conventional soda-lime-silicate and lead-silicate glasses strengthened in the same manner. Also the glass is said to have good working properties suitable for the conventional melting process. This glass is improved as compared with those of Japanese Pat. Publications Nos. 3632/68 and 19420/68 in that it can be sufficiently treated by dipping in a melt of KNO.sub.3 at high temperatures even for much shorter times and has a high strength after the treatment. However, it is necessary that the glass be treated in KNO.sub.3 for more than thirty minutes.
U.S. Pat. No. 2,779,136 to Hood discloses replacing Na.sup.+ or K.sup.+ with Li.sup.+ in a surface layer of SiO.sub.2 -Al.sub.2 O.sub.3 -Na.sub.2 O glass by dipping the glass in a lithium salt at a temperature between the strain point and the softening point of the glass. According to this patent, the strain is generated in the surface layer due to the difference in thermal expansion between the surface layer and the inner body portion of the glass. The alkali metal oxides included in the disclosed glass compositions are substantially Na.sub.2 O and K.sub.2 O; Li.sub.2 O is maintained at less than 1% by weight to prevent the production of .beta.- spodumene in the glass. The working examples indicate that the amount of Li.sub.2 O actually used is less than 0.2% in all instances.
According to this patent, the surface compression strain is produced by the difference of the heat expansion coefficient between the surface and inner portions of the glass.
In the case that crystals of low heat expansion coefficient are educed in the surface portion, it causes a large difference of heat coefficient and a high strength glass is produced. However, this glass has a defect that it is relatively readily devitrified in its surface portion.
To the contrary, when crystals low heat expansion coefficient are not educed in the surface portion, its strength is lower, generally below 3000 kg/cm.sup.2 as the difference in the thermal coefficient due to the alkaline ion exchange is small.
Furthermore, this method has a disadvantage that glass articles are apt to be deformed during the ion exchanging process because the ion exchange is performed at a higher temperature than the distortion point. It has been found that such glass compositions cannot be suitably treated according to the present invention due to he Li.sup.+ content suitable for replacement by Na.sup.+ or K.sup.+.
British Pat. 1,018,890 describes lithium-soda-alumina-silica glass that includes from 2 to 15% ZnO. Sodium fluoride is an optional ingredient and may also be present to the extent of not more than 1%, provided that al least 2% ZnO is also present. As is known in the art, lithium-soda-alumina-silica glass including a high ZnO content is apt to devitrify when the lithium content is increased.
Accordingly, when this glass is treated in the melted salt including sodium ion, a sufficiently high strength is not obtained of the glass. Furthermore, this glass has a high viscosity, and has a disadvantage that it is difficult to obtain homogeneous glass.
Therefore, many attempts have beem made for obtaining glasses which can be treated at a higher rate in the alkali metal ion exchange than in any of the aforementioned glasses, and which have a higher strength even after a short treatment time than any conventional treated soda-lime-silicate and lead-silicate glasses. Also, the compressive surface layer should be of a large thickness and the glass should not deviate so largely in strength from the average strength as to the standard deviation.
An object of the present invention is, therefore, to provide a glass which meets the above requirements.